1. Technical Field
The present invention relates to an optical pickup device in which a working distance is set corresponding to an optical recording medium having a larger substrate thickness to prevent an objective lens from colliding with each optical recording medium during selective recording or reproducing of at least two or more types of optical recording mediums having different substrate thicknesses using an objective lens with a numerical aperture (NA) of 0.75 or more.
2. Background Art
In general, disc-shaped and card-shaped optical recording mediums such as an optical disc and an optical card have been frequently used, because desired tracks are accessible at a high rate in recording information signals such as video information, sound information, and computer data in spirally or concentrically formed tracks on a transparent substrate with high density and in reproducing a recorded track.
As an optical disc constituting this type of optical recording medium, for example, a compact disc (CD), a digital versatile disc (DVD), or the like has been already on the market. In recent years, in order to further increase density of the optical disc, an extra-high density optical disc (Blu Ray Disc) has been well developed, which is capable of recording or reproducing the information signals at an extra-high density as compared with the above-described CD, DVD.
First, in the CD, a laser beam formed by focusing laser light having a wavelength of about 780 nm with an objective lens having a numerical aperture (NA)=about 0.45 has heretofore been applied to a disc substrate to record or reproduce the information signal on or from a signal surface distant from a laser beam incidence surface of a disc substrate approximately by 1.2 mm.
Moreover, in the DVD, the laser beam formed by focusing the laser light having a wavelength of about 650 nm with the objective lens having a numerical aperture (NA)=about 0.6 has heretofore been applied to the disc substrate to record or reproduce the information signal on or from the signal surface distant from the laser beam incidence surface of the disc substrate approximately by 0.6 mm. In this case, a recording capacity of DVD is increased to six to eight times that of CD, and is about 4.7 gigabytes (GB) on one surface, when the disc substrate has a diameter of 12 cm.
Furthermore, the above-described extra-high density optical disc has been developed in such a manner that the laser beam obtained by focusing the laser beam having a wavelength of 450 nm or less with the objective lens having a numerical aperture (NA) of 0.75 or more is applied onto the disc substrate and that the information signals can be recorded on or reproduced from the signal surface distant from a laser beam incidence surface by about 0.05 mm to 0.15 mm. In this case, the recording capacity of one surface of the extra-high density optical disc is around 25 gigabytes (GB), when the disc substrate has a diameter of 12 cm.
Additionally, there is an optical pickup device which records or reproduces three types of optical discs having different disc substrate thicknesses to the signal surface from the laser beam incidence surface (see, for example, “Thoroughly Analyze Next Generation Optical Disc Technique (No. 2), Nikkei Electronics 2003. 5. No. 12, pp. 131 to 133).
In the “Thoroughly Analyze Next Generation Optical Disc Technique (No. 2)”, as a related art of the optical pickup device which records or reproduces three types of optical discs having different disc substrate thicknesses, a double-lens system using two objective lenses (example of Samsung Electronics Co., Ltd., Korea), and a single-lens system using one objective lens (example of LG Electronics Inc., Korea) are described.
Additionally, in the double-lens system described in the document, a CD/DVD combined-use objective lens and an objective lens for Blu-ray disc are arranged on the same base. According to this example, optical adjustment peculiar to the Blu-ray disc can be performed independently with respect to a CD/DVD system. However, since both the objective lenses are controlled in a focusing direction and a tracking direction with one actuator, the weight of a movable portion of the actuator increases, and it is remarkably difficult to realize an actuator of a high band corresponding to a high transfer rate. Moreover, since two objective lenses are used, there is also a cost problem.
On the other hand, in the single-lens system described in the document, the objective lens having a single lens and a high numerical aperture (NA) of 0.85 is used in order to secure a working distance set between the objective lens and each optical disc, but any concrete numerical value example is not described with respect to each working distance for each objective lens.